In an electrophotographic image forming apparatus such as a laser printer, an electrostatic copying machine, a plain paper facsimile machine or a printer-copier-facsimile multifunction machine, an electrostatic latent image formed on a surface of a photoreceptor body by electrically charging the photoreceptor surface and exposing the photoreceptor surface to light is developed into a toner image with a toner, and a developing roller is used for the development.
For the development of the electrostatic latent image into the toner image by the developing roller, the developing roller is rotated in contact with an amount regulating blade (charging blade) in a developing device containing the toner.
Thus, the toner contained in the developing device is triboelectrically charged and applied onto an outer peripheral surface of the developing roller. At the same time, the amount of the applied toner is regulated by the amount regulating blade, whereby a toner layer is formed on the outer peripheral surface of the developing roller as having a generally constant thickness.
When the developing roller is further rotated in this state to transport the toner layer to the vicinity of the surface of the photoreceptor body, the toner of the toner layer is selectively transferred from the toner layer to the surface of the photoreceptor body according to the electrostatic latent image formed on the surface of the photoreceptor body. Thus, the electrostatic latent image is developed into the toner image.
In recent years, a toner including more uniform, more spherical and smaller size toner particles is increasingly used for forming a higher quality image by the image forming apparatus.
Where a toner including highly spherical toner particles is used, however, the friction between the developing roller and the amount regulating blade is reduced during the formation of the toner layer on the surface of the developing roller, thereby reducing the triboelectric charging efficiency. This may result in insufficient charging, so that a formed image is liable to have a reduced image density or suffer from fogging in a margin thereof.
For prevention of these inconveniences, it is conceivable to increase the contact pressure of the amount regulating blade as described, for example, in Patent Document 1. In this case, however, the frictional heat is increased, so that the toner is liable to adhere (fuse and stick) to the surface of the developing roller and a distal edge of the amount regulating blade. The adhesion of the toner may result in white streaking (density unevenness) in the formed image.
In recent years, there is a trend that the toner fixation temperature is set at a lower level to reduce the power consumption of the image forming apparatus, and a toner having a lower melting point for proper fixation at a lower temperature is increasingly used. However, the use of the lower-melting-point toner is more liable to cause the adhesion of the toner, resulting in the density unevenness.
Patent Document 2 proposes to provide a toner capturing/collecting section for capturing and collecting finely broken toner particles liable to adhere to the amount regulating blade, so that the adhesion of the finely broken toner particles can be suppressed.
However, this proposal is not effective for the toner yet to be finely broken. Therefore, it is impossible to prevent the adhesion of the highly spherical toner particles and the lower-melting-point toner particles.
Further, Patent Document 3 proposes to impart the developing roller with a heat conductivity of not less than 0.15 W/m·K to increase the heat releasability of the developing roller, whereby the increase in the surface temperature of the developing roller is suppressed to prevent the adhesion of the toner during the driving of the image forming apparatus.